1. Field of the Invention
The present invention relates to a control device for a lockup clutch for controlling the speed ratio of a torque converter of an automatic transmission in accordance with the operational state of a vehicle.
2. Description of the Related Art
Such a control device is known, for example, from Japanese Patent Application Laid-open No.5-231531. With this control device, an engage pressure is previously gradually increased by a feed-forward control before the engagement of the lockup clutch is started, and when the engagement of the lockup clutch is started, the gradually increased engage pressure is maintained for a given time and then, feedback-controlled such that the actual speed ratio of the torque converter is converged to a target speed ratio, thereby preventing the generation of a shock upon the engagement of the lockup clutch.
However, the known device suffers from a problem that at the start of the engagement of the lockup clutch, it is difficult to retain a clutch piston at an engagement starting position (a position just before a facing surface of the clutch piston abuts against an inner surface of a torque converter cover, or a position in which the facing surface of the clutch piston extremely slightly abuts against the inner surface of the torque converter cover). For this reason, the timing of the engagement is displaced, resulting in a reduced responsiveness.